Use getter for kernel instance

Helps with case where kernel is restarted and a new kernel
instance used.

Ref jupyter#141
Ref jupyter#196

(c) Copyright IBM Corp. 2016

nb-extension/js/init/init.js

@@ -176,7 +176,16 @@ define([
         Urth.suppressErrors = config.suppressErrors;
 
         Urth.events = config.events;
-        Urth.kernel = IPython.notebook.kernel;
+
+        // specify a getter for the kernel instance, since it can be restarted and a new kernel
+        // instantiated
+        Object.defineProperty(Urth, 'kernel', {
+            get: function() {
+                // TODO What is the correct way of handling this outside of the notebook? What
+                //      should we do when using jupyter-js-services and kernel is restarted?
+                return IPython.notebook.kernel;
+            }
+        });
 
         isServerExtensionAvailable(baseURL + COMPONENTS_DIR, function(isAvailable){
             console.log('Server extension is ' + (isAvailable ? '' : 'NOT ') + 'available!');